Fine grinding, pulverizing or size reduction all terms used to denote comminution of relatively soft or resilient materials such as rubbers and plastics has been the subject of a great deal of interest and research for many years. In particular the rubber and plastics industry has been interested in fine grinding scrap or reject material in order to recycle the material for reuse. However, in order to make effective use of recycle material the recycle material must be ground to a particle size finer than 80 mesh in order to be mixed with virgin material for reuse.
An effective and economically cost justifiable fine grinding system would be an asset to the rubber and plastics industry because of the various states and the federal government making it more and more difficult to dispose of scrap rubber and plastic material. Currently there is no economical process for producing powdered rubber or plastic of a particle size finer than 80 mesh. Thus large amounts of scrap and reject material are being stockpiled for recycle or disposition in accordance with current environmental laws.
In the past several attempts have been made to solve the problem of fine grinding by utilizing different types of grinding (pulverizing, size reducing or comminution) mills which incorporate a liquid cryogen such as liquid nitrogen either before or inside the mill.
For example conventional hammer mills have utilized liquid nitrogen both inside the mill and before the mill in a pre-cooler. However over the past decade within which these devices have been used it has been found that the hammer mill cannot grind the tough rubber and plastics economically because of the excessive amount of liquid nitrogen required.
Attrition mills have also been modified to use liquid nitrogen both inside and before the mill in a pre-cooler. This also has been done for about 10 years and it is known that the attrition mill cannot grind the tough rubber and plastics economically due to excessive plate wear inside the mill.
Lastly air-swept impact mills (fluid classification mills) have been utilized with nitrogen to pre-cool the material to be ground. Although a fine ground material was obtained the economics of the process were unfavorable due to the excessive amounts of liquid nitrogen required.
The art is replete with various types of schemes to utilize liquid nitrogen in combination with a grinding mill to effect the grinding of normally resilient or soft materials. U.S. Pat. Nos. 2,609,150, 2,735,624, 2,919,862, 3,614,001, 3,771,729 and 4,273,294 illustrate the use of a liquid cryogen (liquid nitrogen) to pre-cool and/or cool the grinding mill to achieve a ground product.
U.S. Pat. 3,771,729 shows a particularly effective pre-cooling apparatus for conditioning the material for grinding.
Other techniques for grinding materials that are normally soft, viscous, plastic or elastomeric in nature at room temperature are illustrated in U.S. Pat. Nos. 441,951, 637,465, 2,467,318, 3,314,802, 2,347,464, 2,435,503, 2,583,697, 3,647,149, 3,658,259, 2,665,850, 3,734,412, 2,836,368, 2,893,216, 2,974,883, 3,319,895.